Project 1 (Crotty) Helper T cell differentiation and function are important processes for many diseases. Vaccines are one of the most cost effective medical treatments in modern civilization. The vast majority of current human vaccines function by eliciting protective antibody responses. T cell help to B cells is a fundamental aspect of adaptive immunity to many pathogens. Follicular helper CD4 T cells (Tfh) are the specialized providers of help to B cells. Therefore, there is substantial potential for an improved understanding of Tfh cells to facilitate better anti- pathogen immune responses and vaccine-elicited humoral immunity. Work by our laboratory and others established that TFH cells depend on expression of the transcription factor Bcl6 and other transcription factors. Despite these advances, the pathways that control TFH differentiation and define TFH functions remain poorly understood. In Project 1, we will characterize, stratify, and interconnect TFs, chromatin regulators, and helper molecules that control TFH differentiation and function, leveraging our current tools and our team?s knowledge of related pathways in CD8 T cells (Projects 2 & 3). Aim 1. To understand the mechanisms of action of Bcl6 in TFH cell differentiation and function. Bcl6 is the lineage defining transcription factor of TFH cells. How Bcl6 accomplishes control of TFH differentiation and function remains unclear, because of the complexity of the biology. Based on preliminary data, our operating model is that Bcl6 controls TFH differentiation by repressor-of- repressor mechanisms. Putative mechanisms will be comprehensively tested. Aim 2. Aim 1 has identified and will identify key Bcl6-r TFs. In Aim 2 we explore the biology of these TFs and the putative mechanisms by which these TFs control TFH. Aim 3. TFH help to B cells is a multifaceted process for which much is still unknown regarding the molecules involved. This is in part because it is a complex set of functions, and in part because of previous technological limitations. It is likely that elucidating the immunobiology underlying the differentiation of Tfh cells and the process of generating protective antiviral antibody responses will reveal vaccinology principles that can be applied to future vaccine development against infectious scourges.